A novel mutated acetolactate synthase gene conferring specific resistance to pyrimidinyl carboxy herbicides in rice

Acetolactate synthase (ALS) is the first common enzyme in the biosynthetic pathway of branched-chain amino acids. Mutations of specific amino acids in ALS have been known to confer resistance to ALS-inhibiting herbicides such as sulfonylureas and pyrimidinyl carboxy (PC) herbicides. However, mutations conferring exclusive resistance to PC have not yet been reported to date. We selected PC resistant rice calli, which were derived from anther culture, using one of the PCs, bispyribac-sodium (BS), as a selection agent. Two lines of BS-resistant plants carrying a novel mutation, the 95th Glycine to Alanine (G95A), in ALS were obtained. In vitro ALS activity assay indicated that the recombinant protein of G95A-mutated ALS (ALS-G95A) conferred highly specific resistance to PC herbicides. In order to determine if the ALS-G95A gene could be used as a selection marker for rice transformation, the ALS-G95A gene was connected to ubiquitin promoter and introduced into rice. PC resistant plants containing integrated ALS-G95A gene were obtained after selection with BS as a selection agent. In conclusion, novel G95A mutated ALS gene confers highly specific resistant to PC-herbicides and can be used as a selection marker.     

Camelina mutants resistant to acetolactate synthase inhibitor herbicides

Camelina (Camelina sativa L.) is a low-input oilseed crop of recent interest for sustainable biofuel production. As a relatively new crop in modern agriculture, considerable agronomic and regulatory problems need to be overcome. A common and troublesome problem is sensitivity to residues of acetolactate synthase (ALS) inhibitor herbicides in soils. To develop resistance to those residues, camelina seed were mutagenized by exposure to 0.3% ethyl methane sulfonate and screened at the M2 generation for increased resistance to imazethapyr and sulfosulfuron. Five lines with resistance were identified and characterized. Four mutants, identified in a screen for imazethapyr resistance (IM1, IM6, IM10, and IM18), appeared phenotypically identical and were controlled by the same co-dominant gene. One mutant identified in a screen for sulfosulfuron resistance was phenotypically different but also appears to be controlled by a single co-dominant gene. Further analysis with the IM1 and SM4 mutants confirmed they had increased resistance to imazethapyr, sulfosulfuron, and flucarbazone, with the resistance in the SM4 mutant being the highest. Compared to the wild type, doses of approximately 200 times more imazethapyr, 30 times more sulfosulfuron, and seven times as much flucarbazone were required to reduce plant growth by 50%. Sequence analysis of ALS genes from the SM4 line identified at least eight different genes or alleles. An allele associated with the highest levels of resistance was created by a single base substitution creating an amino acid shift previously found to cause ALS inhibitor resistance in yeast and tobacco.     

Amino acid residues conferring herbicide tolerance in tobacco acetolactate synthase

Acetolactate synthase (ALS) is the common enzyme in the biosynthetic pathways leading to valine, leucine, and isoleucine in plants and microorganisms. ALS is the target site of several classes of structurally unrelated herbicides including sulfonylureas, imidazolinones, and triazolopyrimidines. To identify the residues conferring herbicide tolerance in tobacco ALS, site-directed mutagenesis for three residues, Ala121, Pro187 and Ser652, was performed. Mutant A121T showed strong resistance to Londax (a sulfonylurea) and Cadre (an imidazolinone), while mutant S652T was resistant only to Cadre. The S652N mutation abolished the binding affinity of FAD, and inactivated the enzyme. Double mutation of Ala121 and Ser652 with Thr yielded a mutant highly tolerant to Londax, Cadre, and TP (a triazolopyrimidine sulfonamide), but has enzymatic properties similar to those of wild-type. Substitution of Pro187 with Ser resulted in the enzyme highly susceptible to oxidation and fragmentation. These results suggest that two residues Ala121 and Ser652 are potent residues conferring herbicide resistance in tobacco ALS, and that double mutation of Ala121 and Ser652 by Thr can confer stronger tolerance to Londax, Cadre, and TP.     

Transformation with a mutant Arabidopsis acetolactate synthase gene renders tobacco resistant to sulfonylurea herbicides

Summary A gene encoding acetolactate synthase was cloned from a chlorsulfuron-resistant mutant of Arabidopsis. The DNA sequence of the mutant gene differed from that of the wild type by a single base pair substitution. When introduced into tobacco by Ti plasmid-mediated transformation the gene conferred a high level of herbicide resistance. These results suggest that the cloned gene may confer agronomically useful levels of herbicide resistnace in other crop species, and that it may be useful as a selectable marker for plant transformation experiments.     

Roles of conserved methionine residues in tobacco acetolactate synthase

Acetolactate synthase (ALS) catalyzes the first common step in the biosynthesis of valine, leucine, and isoleucine. ALS is the target of several classes of herbicides, including the sulfonylureas, the imidazolinones, and the triazolopyrimidines. The conserved methionine residues of ALS from plants were identified by multiple sequence alignment using ClustalW. The alignment of 17 ALS sequences from plants revealed 149 identical residues, seven of which were methionine residues. The roles of three well-conserved methionine residues (M350, M512, and M569) in tobacco ALS were determined using site-directed mutagenesis. The mutation of M350V, M512V, and M569V inactivated the enzyme and abolished the binding affinity for cofactor FAD. Nevertheless, the secondary structure of each of the mutants determined by CD spectrum was not affected significantly by the mutation. Both M350C and M569C mutants were strongly resistant to three classes of herbicides, Londax (a sulfonylurea), Cadre (an imidazolinone), and TP (a triazolopyrimidine), while M512C mutant did not show a significant resistance to the herbicides. The mutant M350C was more sensitive to pH change, while the mutant M569C showed a profile for pH dependence activity similar to that of wild type. These results suggest that M512 residue is likely located at or near the active site, and that M350 and M569 residues are probably located at the overlapping region between the active site and a common herbicide binding site.     

Role of tryptophanyl residues in tobacco acetolactate synthase.

Acetolactate synthase (ALS) catalyzes the first common step in the biosynthesis of valine, leucine, and isoleucine. ALS is the target of three classes of herbicides, the sulfonylureas, the imidazolinones, and the triazolopyrimidines. Five mutants (W266F, W439F, W490F, W503F, and W573F) of the ALS gene from Nicotiana tabacum were constructed and expressed in Escherichia coli, and the enzymes were purified. The W490F mutation abolished the binding affinity for cofactor FAD and inactivated the enzyme. The replacement of Trp573 by Phe yielded a mutant ALS resistant to the three classes of herbicides. The other three mutations, W266F, W439F, and W503F, did not significantly affect the enzymatic properties and the sensitivity to the herbicides. These results indicate that the Trp490 residue is essential for the binding of FAD and that Trp573 is located at the herbicide binding site. The data also suggest that the three classes of herbicides bind ALS competitively.     

Oxidative stress is not related to the mode of action of herbicides that inhibit acetolactate synthase

Imazethapyr (IM) is an imidazolinone herbicide which inhibits the biosynthesis of branched chain amino acids, by blocking acetolactate synthase (ALS; EC 4.1.3.18), the first common enzyme of the pathway. To study new aspects of the mode of action of ALS-inhibiting herbicides, pea plants grown in hydroponic cultures were supplied with IM and were analysed with reference to the antioxidant system and oxidative markers. A slight lipid peroxidation was detected in leaves after IM treatment, but no changes were noted in electrolyte leakage or carbonyl content. The ascorbate pool of leaves was oxidized under IM treatment. The analysis of the antioxidant enzymes superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione reductase (GR), catalase (CAT) and guaiacol peroxidase (GPX), showed that IM treatment only caused an enhancement of GPX activity in leaves. In roots, the herbicide caused a decrease in lipid peroxidation. The enhancement of the reduced glutathione content detected in IM-treated roots can be related to the detected increase of GR activity. The lack of more noticeable effects on antioxidant enzymatic activities could be explained by the inability of IM-treated plants to respond to oxidative stress with modifications in their protein synthesis. Our results suggest that oxidative stress is not related to the mode of action of ALS-inhibitors. The slight changes detected in the antioxidative status of treated plants are too secondary in time and intensity to be related to the lethality caused by ALS-inhibitors     

Inheritance and mechanism of resistance to herbicides inhibiting acetolactate synthase in Sonchus oleraceus L.

A biotype of Sonchus oleraceus L. (Compositae) has developed resistance to herbicides inhibiting acetolactate synthase (ALS) following field selection with chlorsulfuron for 8 consecutive years. The aim of this study was to determine the inheritance and mechanism of resistance in this biotype. Determination of ALS activity and inhibition kinetics revealed that K_m and V_max did not vary greatly between the resistant and susceptible biotypes. ALS extracted from the resistant biotype was resistant to five ALS-inhibiting herbicides in an in vitro assay. ALS activity from the resistant biotype was 14 19, 2, 3 and 3 times more resistant to inhibition by chlorsulfuron, sulfometuron, imazethapyr, imazapyr and flumetsulam, respectively, than the susceptible biotype. Hybrids between the resistant and a susceptible biotype were produced, and inheritance was followed through the F₁, F₂ and F₃ generations. F₁ hybrids displayed a uniform intermediate level of resistance between resistant and susceptible parents. Three distinct phenotypes, resistant, intermediate and susceptible, were identified in the F₂ generation following chlorsulfuron application. A segregation ratio of 121 was observed, indicative of the action of a single, nuclear, incompletely dominant gene. F₃ families, derived from intermediate F₂ individuals, segregated in a similar manner. Resistance to herbicides inhibiting ALS in this biotype of S. oleraceus is due to the effect of a single gene coding for a resistant form of the target enzyme, ALS.     

Efficient transformation of wheat by using a mutated rice acetolactate synthase gene as a selectable marker

Acetolactate synthase (ALS) is a target enzyme for many herbicides, including sulfonylurea and imidazolinone. We investigated the usefulness of a mutated ALS gene of rice, which had double point mutations and encoded an herbicide-resistant form of the enzyme, as a selectable marker for wheat transformation. After the genomic DNA fragment from rice containing the mutated ALS gene was introduced into immature embryos by means of particle bombardment, transgenic plants were efficiently selected with the herbicide bispyribac sodium (BS). Southern blot analysis confirmed that transgenic plants had one to more than ten copies of the transgene in their chromosomes. Adjustment of the BS concentration combined with repeated selection effectively prevented nontransgenic plants from escaping herbicide selection. Measurement of ALS activity indicated that transgenic plants produced an herbicide-resistant form of ALS and therefore had acquired the resistance to BS. This report is the first to describe a selection system for wheat transformation that uses a selectable marker gene of plant origin.     

Direct sequencing of tobacco chloroplast genome by the polymerase chain reaction

We have developed a polymerase chain reaction (PCR) method for sequencing of tobacco chloroplast genome. In a mixture containing chloroplast DNA, 5-end-labeled oligonucleotide primer, Taq DNA polymerase and reaction buffer, we were able to sequence a segment of chloroplast 16S rRNA gene. The results showed that the 750 bp of DNA sequenced were identical to the sequence reported, indicating that direct sequencing method that we have developed is useful for the sequencing of chloroplast genome. To analyze the chloroplast genome more rapidly in those in vitro grown plantlets, we also developed a simple method which is applicable for the amplifications and sequencing of chloroplast 16S rRNA fragment from either 0.15 g of tobacco leaf or stem tissue. The readable sequences obtained from the presented methods were consistent with the published sequence.     

Roles of lysine 219 and 255 residues in tobacco acetolactate synthase

Acetolactate synthase (ALS) catalyzes the first common step in the biosynthesis of valine, leucine, and isoleucine. The ALS is the target of several classes of herbicides, including the sulfonylureas, the imidazolinones, and the triazolopyrimidines. The roles of three well-conserved lysine residues (K219, K255, K299) in tobacco ALS were determined using site-directed mutagenesis. The mutation of K219Q inactivated the enzyme and abolished the binding affinity for cofactor FAD. However, the secondary structure of the enzyme was not changed significantly by the mutation. Both mutants, K255F and K255Q, showed strong resistance to three classes of herbicides Londax (a sulfonylurea), Cadre (an imidazolinone), and TP (a triazolopyrimidine). In addition, there was no difference in the secondary structures of wALS and K255F. On the other hand, the mutation of K299Q did not show any significant effect on the kinetic properties or any sensitivity to the herbicides. These results suggest that Lys219 is located at the active site and is likely involved in the binding of FAD, and that Lys255 is located at a binding site common for the three herbicides in tobacco ALS.     

Structural and functional role of cysteinyl residues in tobacco acetolactate synthase

Acetolactate synthase (ALS) is the common enzyme in the biosynthesis of valine, leucine, and isoleucine. The role of four cysteinyl residues in tobacco ALS was determined using site-directed mutagenesis and cysteine-specific cleavage. The C411A mutation abolished the enzymatic activity, as well as the binding affinity for the cofactor FAD. The activation constant of C411S for FAD is approximately 50-fold higher than that of wALS. The C607S mutation did not significantly affect the kinetic parameters. The IC(50) values of C411S and C607S for ALS-inhibiting herbicides are not much different from those of wALS. Two mutants, C163S and C309S, are labile and readily degraded to peptide fragments. The treatment of wALS with 2-nitro-5-thiocyanobenzoic acid, specific for cleavage of the N-terminal side of cysteine, yielded three peptides of 37.0, 22. 0, and 7.0 kDa. This fragmentation pattern is consistent with that deduced from the amino acid sequence of tobacco ALS, assuming the disulfide bond between Cys163 and Cys309. These results suggest that Cys411 is involved in the binding of FAD and that the intrachain disulfide bond between Cys163 and Cys309 plays a key role in maintaining the correct conformation of tobacco ALS.     

A single amino acid change in acetolactate synthase confers resistance to valine in tobacco

The metabolic control of branched chain amino acid (BCAA) biosynthesis involves allosteric regulation of acetolactate synthase (ALS) by the end-products of the pathway, valine, leucine and isoleucine. We describe here the molecular basis of valine resistance. We cloned and sequenced an ALS gene from the tobacco mutant Val^r-1 and found a single basepair substitution relative to the wild-type allele. This mutation causes a serine to leucine change in the amino acid sequence of ALS at position 214. We then mutagenized the wild-type allele of the ALS gene ofArabidopsis and found that it confers valine resistance when introduced into tobacco plants. Taken together, these results suggest that the serine to leucine change at position 214 of ALS is responsible for valine resistance in tobacco.This paper is dedicated to the memory of Jean-Pierre Bourgin, who died on October 29, 1994, at the age of 50     

Bansformation of tobacco with a mutated cyanobacterial phytoene desaturase gene confers resistance to bleaching herbicides

Carotenoids are constituents of the photosynthetic apparatus and essential for plant survival because of their involvement in protection of chlorophylls against photooxidation. Certain classes of herbicides are interfering with carotenoid biosynthesis leading to pigment destruction and a bleached plant phenotype. One important target site for bleaching herbicides is the enzyme phytoene desaturase catalysing the desaturation of phytoene in zeta-carotene. This enzymatic reaction can be inhibited by norflurazon or fluridone. We have transformed tobacco with a mutated cyanobacterial phytoene desaturase gene (pds) derived from the Synechococcus PCC 7942 mutant NFZ4. Characterization of the resulting transformants revealed an up to 58 fold higher norflurazon resistance in comparison to wild type controls. The tolerance for fluridone was also increased 3 fold in the transgenics. Furthermore, the transformed tobacco maintained a higher level of D1 protein of photosystem II indicating a lower susceptibility to photooxidative damage in the presence of norflurazon. In contrast, the genetic manipulation did not confer herbicide resistance against zeta-carotene desaturase inhibitors.     

Positioning of protein-coding genes on the soybean chloroplast genome

Seven major plastid protein encoding genes were positioned on the soybean chloroplast DNA by heterologous hybridization. These include the genes for the alpha, beta and epsilon subunits of the CF₁ component of ATP synthase (atpA, atpB and atpE respectively), for subunit III of the CF₀ component of ATP synthase (atpH), for the cytochrome f (cytF), for the ‘32 Kd’ thylakoid protein (psbA), and for the large subunit of ribulose-1,5-bisphosphate carboxylase-oxygenase (rbcL), all of which map in the large single copy region. The atpB, atpE and rbcL genes are located in the region adjacent to one of the segments of the inverted repeat. The genetic organization of the soybean chloroplast DNA is compared to that of other plastid genomes.